1. Field of the Invention
The present invention relates to a Broadband Wireless Access (BWA) system. More particularly, the present invention relates to an apparatus and method for Connection Admission Control (CAC) considering traffic characteristics.
2. Description of the Related Art
In a wireless access system, when limited network resources are available for distribution to a plurality of user terminals, the number of access connections (or the amount of traffic) must be restricted to ensure Quality of Service (QoS). To this end, Connection Admission Control (CAC) is used to determine whether to admit or deny access by a terminal that is requesting connection (i.e., a connection admission requesting connection) or requesting a connection for a QoS requirement change (e.g., a per-flow mode change) in consideration of currently available resources.
FIG. 1 is a diagram illustrating a general concept of a conventional CAC.
According to a conventional CAC algorithm, when a connection admission request is received from a user terminal, a wireless access system estimates an available channel capacity (having the same unit as a bandwidth or a transmission rate). Thereafter, the wireless access system compares the estimated available channel capacity with a bandwidth requirement for input traffic (i.e., a transmission request rate for a connection corresponding to the connection admission request of the user terminal) to determine whether to admit or deny the requested connection of the user terminal. For example, if the transmission request rate is less than the available channel capacity, the wireless access system admits the connection of the user terminal. If not, the wireless access system denies the connection of the user terminal. Thus, the core issues of the CAC are to provide a method of estimating the available channel capacity and a method of specifying the bandwidth requirement for input traffic.
In the conventional circuit-based wireless networks (e.g., Code Division Multiple Access (CDMA) and Global System for Mobile communications (GSM)), because a per-connection required capacity and a radio channel capacity are constant, a CAC process is performed in consideration of only the number of currently-served connections. However, in the third-generation or later wireless networks such as the World interoperability for Microwave Access (WiMAX) and the Third-Generation Partnership Project 2 (3GPP2), because the advanced access techniques (e.g., Adaptive Modulation and Coding (AMC), Hybrid Automatic Repeat reQuest (HARQ), Proportional Fair (PF) Scheduling, and Multi Input Multi Output (MIMO)) are used selectively according to the respective traffic characteristics, an available channel capacity changes dynamically and is thus difficult to estimate.
Also, the amount of radio resources used to secure the same required bandwidth varies with the traffic types. For example, although Voice-over-Internet Protocol (VoIP), multimedia streaming, and File Transfer Protocol (FTP) download application programs require the same bandwidth of 64 Kbps, the amounts of radio resources used to serve them differ from each other.
Furthermore, in the advanced wireless network, an air resource allocation method of a downlink (DL) is different from an air resource allocation method of an uplink (UL). Thus, although the QoS types are the same, the amount of air resources used in the DL are different from the amount of air resource used in the UL. For example, for an Unsolicited Grant Service (UGS), the UL allocates air resources periodically regardless of whether traffic has arrived. However, the DL does not allocate air resources in advance, but allocates air resources when traffic has arrived. Therefore, a wider bandwidth must be allocated to the UL than to the DL in consideration of a waste of air resources. In the case of the UL, a waste of air resources must be considered in allocation of radio resources because a bandwidth request process is included for all the QoS types except the UGS. In the case of a real-time Polling Service (rtPS), a DL resource waste occurs due to a polling operation for a UL flow because the UL performs a polling operation but the DL does not perform a polling operation. User terminals have different Modulation and Coding Scheme (MCS) levels due to an AMC function used in the advanced wireless network, although it is a minute difference in overhead. A MAC overhead for the DL can be reduced by scheduling user terminals with the same MCS level in one burst, but it is difficult to do this for the UL, causing a further resource waste due to the overhead. Overall, a waste of DL/UL air resources occurs in a UL flow.
Application programs with various traffic characteristics (e.g., real-time vs. non-real-time, and fixed data rate vs. variable data rate) are present in the third-generation or later wireless networks. Thus, a method for clearly specifying the characteristics of input traffic is required in order to effectively support such application programs.